The present invention relates generally to steam plants having at least one intermediate superheater.
In conventional power plants having turbine systems containing interstage superheaters and by-pass arrangements of large capacity, a pressure reduction in the superheater will take place relatively slowly after a load cut-off at the turbine, followed by a fully opened by-pass. For example, boilers heated by coal and not shut down in the case of a load cut-off at the turbine, will have a load dissipation gradient (in minutes) of 5%/minute. In the case of a proper load dissipation at the boiler, it is likely that the pressure in the intermediate superheater will require at least 15 minutes before being reduced so as to correspond to the pressure of minimum boiler load.
If the turbine plant is operated at no load or internal power only (with a higher pressure in the intermediate superheater), a limited amount of steam will flow through the high-pressure turbine. This limited amount of steam will usually not be sufficient to dissipate windage heat generated due to the pressure build-up in the intermediate superheater. The excess windage heat may in turn possibly cause an uncontrollable temperature rise in the exhaust steam of the high-pressure turbine. Alternatively, the excess windage heat may cause a limited idling capacity. Therefore, between the moment of load cut-off and the occurrence of the necessary lower pressure in the intermediate superheater there will be a period of time during which the temperature is out-of-control with the potential danger of serious damage occurring within the turbine plant.
Turbine plants which are equipped with interstage superheaters and both high- and low-pressure by-passes have the further disadvantage of an extended starting period in the case of a cold start. The extended starting period occurs because a sudden rise in temperature will occur at the very beginning of the turbine run which is caused by the pressure built up in the intermediate superheater in accordance with the boiler load and which pressure build up will attenuate the acceleration gradient.
Attempts have been made to alleviate these problems (i.e., a proper idling capability after a load cut-off and optimum cold start behavior) by an alignment of the turbine relief valves relative to the intake valves or by some other measures. These procedures, however, fail to lead to an overall solution which would be satisfactory even in a marginal situation.
Accordingly, it is a primary object of the present invention to provide a steam turbine plant which does not have the disadvantages of the present-day plants discussed above, wherein (even in the case of a by-pass capacity of up to 100%) a load cut-off at the turbine will not require a tripping of the boiler.
Another object of the present invention is to provide a steam turbine plant wherein the plant can be reduced properly to a minimum load and operated under these conditions at no load or under internal power for any desired period of time. In this way, the unit will be ready immediately, e.g., after the elimination of a fault in the power system.
Finally, still another object of the present invention is to provide a steam turbine plant having a relatively shorter cold-starting time than the known steam turbine plants due to the simultaneous heating of the high-pressure and the medium-pressure turbines.
A steam turbine plant according to the present invention includes a discharge pipe line having a flow-regulating unit which by-passes the intermediate superheater. One end of the discharge pipe line is connected on a line between the check valve and the high-pressure turbine on the outlet side of the latter, and the other end of the discharge pipe line is connected with the intake side of a condenser.
In a further embodiment of the present invention, it is desirable to provide at least one high-pressure feed water heater between the feed tank and the boiler. The high-pressure feed water heater is connected at the steam side between the high-pressure turbine and the check valve (which check valve is arranged between the high-pressure turbine and the intermediate superheater on the outlet side of the high-pressure turbine) by way of a high-pressure steam conduit. As a result of this arrangement, the feed water temperature will be automatically lowered to a certain degree so that the boiler will be able to reduce the load more rapidly. Furthermore, in the case of this particular embodiment, the steam (prohibited by the closed check valve from entering the intermediate superheater) will reach the high-pressure feed water heater so that the conditions of heat balance will not be affected significantly so far as the boiler is concerned.
In connection with this arrangement it will also be advantageous to connect the flow-regulating unit (located within the discharge pipe line) to a control device. The control device is preferably arranged to maintain the flow-regulating unit in a closed configuration during the start of the turbine plant until a pre-set discharge pressure has been reached by the high-pressure turbine. The control device will then regulate the open position of the flow-regulating unit is such a manner that the pressure within the high-pressure turbine will not exceed the predetermined pressure value.
At the time of parallel connection of the generator with the power network, the open position of the flow-regulating unit is held either until a pre-set minimum threshold load value has been reached, or until the pressure at the inlet and outlet sides of the check valve (arranged between the high-pressure turbine and the intermediate superheater) has been approximately equalized. The flow-regulating unit will be closed when the turbine load increases still further.